Extended focus control

ABSTRACT

An apparatus for scanning a track on a record carrier has a head ( 41 ) for providing a beam of radiation and generating at least one sensor signal. The device has a focusing unit for controlling a focusing element to maintain an in-focus position for generating a focused spot on the track. The sensor signal is converted into a displacement signal by a multitude of piecewise converters ( 42, 43, 44 ), each converter converting a range of the sensor signal values into a position signal, and selection means ( 46 ) for selecting one of the position signals as the displacement signal based on an estimated displacement.

The invention relates to a device for scanning a track on a recordcarrier via a beam of radiation, the track comprising marks representinginformation, the device comprising a head for providing the beam andgenerating at least one sensor signal, the head comprising a focusingelement, a focusing unit for controlling the focusing element tomaintain an in-focus position for generating a focused spot on the trackin dependence on a displacement signal based on the sensor signal, thedisplacement signal being indicative of a displacement of the focusingelement with respect to an in-focus position, the sensor signal havingoverlapping ranges of sensor signal values for different ranges of thedisplacement.

The invention further relates to a method of scanning a track on arecord carrier via a beam of radiation, the track comprising marksrepresenting information, the method comprising generating at least onesensor signal, controlling a focusing element to maintain an in-focusposition for generating a focused spot on the track in dependence on adisplacement signal based on the sensor signal, the displacement signalbeing indicative of a displacement of the focusing element with respectto an in-focus position, the sensor signal having overlapping ranges ofsensor signal values for different ranges of the displacement.

A device and method for scanning a track and reading information areknown from U.S. Pat. No. 6,314,069. The device has a head for providinga beam of radiation focused on a track. A read signal is generated forreading information represented by marks in a track on a record carrierlike an optical disc. An error signal representing a displacement of afocusing element from an in-focus position is produced based on twodetector signals from a detector. The detector signals are presumed tohave a periodical characteristic relative to the displacement. A servosystem responsive to the error signal controls the focusing element tobring the beam to the in-focus position. A local feedback loop iscoupled to the detector signals, and the error signals (that are shiftedin phase) are coupled via two periodic function generators to twomultipliers, which multiply the detector signals by the output of theperiodic function generators. The output of the multipliers is coupledto the circuit to modify the error signal. As a result the operationalrange of the focus servo loop is extended to a range of displacementsoutside a limited working range around the nominal in-focus position. Aproblem of the known device is that two out-of-phase detector signalshaving a periodic characteristic are required, and that the focus rangeextension of displacements outside the limited working range is notaccurate.

Therefore it is an object of the invention to provide a device andmethod for scanning a record carrier having an accurate extended focusrange.

According to a first aspect of the invention the object is achieved witha device as defined in the opening paragraph, characterized in that thedevice comprises a multitude of piecewise converters, each converterconverting a range of the sensor signal values into a position signal,and selection means for selecting one of the position signals as thedisplacement signal based on an estimated displacement.

According to a first aspect of the invention the object is achieved witha method as defined in the opening paragraph, the method comprisingconverting a multitude of ranges of the sensor signal values into amultitude of position signals, and selecting one of the position signalsas the displacement signal based on an estimated displacement.

The effect of the piecewise converting is that relevant ranges of sensorsignal values are converted by dedicated converters. The selection ofthe relevant position signal is based on the estimated displacement.This has the advantage that operation of the focus servo system isextended more accurately outside a substantial linear range around thein-focus position, in particular including non-linear ranges havingoverlapping sensor signal values.

The invention is also based on the following recognition. In prior artsystems the focusing system is usually arranged for a substantiallylinear working region around a nominal position. Outside the linearrange, i.e. for larger displacements, the sensor signal decreasesconstituting inverted and relatively long tail parts of the curve; thetotal curve usually being called s-curve. The inventors have seen that,although for existing optical disc systems the linear region providessufficient working range, in high density small form factor opticaldrives the linear range is relatively small. Hence the drive would bevery sensitive to shock, and extension of the working range outside thelinear region is attractive. Actually the tail parts cover a range ofdisplacements significantly larger than the linear part. Presuming aperiodic characteristic as in the known s-curve extension in U.S. Pat.No. 6,314,069 is highly inaccurate and unduly limits the extension toabout twice the size of the linear region.

In an embodiment of the device the selection means comprise an estimatorfor determining the estimated displacement based on previous values ofthe displacement signal. The effect is that movement of the focusingelement with respect to the track is assumed to continue from theprevious samples. This has the advantage that the selection of aposition signal is based on a realistic physical model of the focusingelement.

In an embodiment of the device at least one of the piecewise convertershas a substantially proportional relation between the range of thesensor signal values and the position signal, and at least one of thepiecewise converters has a substantially inverse relation between therange of the sensor signal values and the position signal. Thereby theposition signal provides displacement values of the focusing elementoutside the limited linear region of the s-curve. This has the advantagethat an inverting function provides a simple and effective translationof a tail of the s-curve to a position signal.

Further preferred embodiments of the device according to the inventionare given in the further claims.

These and other aspects of the invention will be apparent from andelucidated further with reference to the embodiments described by way ofexample in the following description and with reference to theaccompanying drawings, in which

FIG. 1 a shows a record carrier (top view),

FIG. 1 b shows a record carrier (cross section),

FIG. 1 c shows a detail of a track,

FIG. 2 shows a scanning device having a focus extension,

FIG. 3 shows a sensor signal of a focus sensor,

FIG. 4 shows a focus extension unit,

FIG. 5 shows displacement of an objective lens,

FIG. 6 shows a sensor signal,

FIG. 7 a shows position signals for the tail parts of the s-curve,

FIG. 7 b shows a position signal for the central part of the s-curve,

FIG. 7 c shows a combination of position signals, and

FIG. 8 shows a reconstructed displacement signal.

In the Figures, elements which correspond to elements already describedhave the same reference numerals.

FIG. 1 a shows a disc-shaped record carrier 11 having a track 9 and acentral hole 10. The track 9, being the position of the series of (tobe) recorded marks representing information, is arranged in accordancewith a spiral pattern of turns constituting substantially paralleltracks on an information layer. The record carrier may be opticallyreadable, called an optical disc, and has an information layer of aread-only or recordable type. Details about the read-only DVD disc canbe found in reference: ECMA-267: 120 mm DVD—Read-Only Disc—(1997).Examples of a recordable disc are the CD-R and CD-RW, and writableversions of DVD, such as DVD+RW, and the high density writable opticaldisc using blue lasers, called Blue-ray Disc (BD). The information isrepresented on the information layer by providing optically detectablemarks along the track, e.g. pits or crystalline or amorphous marks inphase change material. The track 9 on the recordable type of recordcarrier is indicated by a pre-embossed track structure provided duringmanufacture of the blank record carrier. The track structure isconstituted, for example, by a pregroove 14 which enables a read/writehead to follow the track during scanning. The track structure comprisesposition information, e.g. addresses.

FIG. 1 b is a cross-section taken along the line b-b of the recordcarrier 11 of the recordable type, in which a transparent substrate 15is provided with a recording layer 16 and a protective layer 17. Theprotective layer 17 may comprise a further substrate layer, for exampleas in DVD where the recording layer is at a 0.6 mm substrate and afurther substrate of 0.6 mm is bonded to the back side thereof. Thepregroove 14 may be implemented as an indentation or an elevation of thesubstrate 15 material, or as a material property deviating from itssurroundings.

In an embodiment the record carrier 11 is carrying informationrepresenting digitally encoded video according to a standardized formatlike MPEG2.

FIG. 1 c shows an example of a wobble of the track. A detail 12 of thetrack 9 shows a periodic variation of the lateral position of thepregroove 14, also called wobble. The variations cause an additionalsignal to arise in auxiliary detectors, e.g. in the push-pull channelgenerated by partial detectors in the central spot in a head of ascanning device. The wobble is, for example, frequency modulated andposition information is encoded in the modulation. A comprehensivedescription of the wobble and encoding information therein can be foundfor CD in U.S. Pat. No. 4,901,300 (PHN 12.398) and U.S. Pat. No.5,187,699 (PHQ 88.002), and for the DVD+RW system in U.S. Pat. No.6,538,982 (PHN 17.323).

FIG. 2 shows a scanning device having a focus extension. The device isprovided with means for scanning the track on record carrier 11 whichinclude a drive unit 21 for rotating the record carrier 11, a head 22, apositioning unit (not shown) for coarsely positioning the head 22 in theradial direction on the track and a focusing unit 25 for focusing aradiation beam on the track, and a control unit 20. The head 22comprises an optical system of a known type for generating the radiationbeam 24 guided through optical elements focused to a radiation spot 23on a track of the information layer of the record carrier. The radiationbeam 24 is generated by a radiation source, e.g. a laser diode. The headcomprises a focusing element 34, for example an objective lens,controlled by the focusing unit via an actuator for moving the focus ofthe radiation beam 24 along the optical axis of said beam. The headfurther comprises a tracking actuator (not shown) for fine positioningof the spot 23 in a radial direction on the center of the track. Thetracking actuator may comprise coils for radially moving an opticalelement or may alternatively be arranged for changing the angle of areflecting element. For reading the radiation reflected by theinformation layer is detected by a detector of a usual type, e.g. afour-quadrant diode, in the head 22 for generating detector signalscoupled to a front-end unit 31 for generating various scanning signals,including a main scanning signal 33 and error signals 35 for trackingand focusing. The error signals 35 are coupled a focusing extension unit32 to generate a displacement signal 37 which is coupled to the focusunit 25 for controlling said focusing actuators. The error signals 35may also be coupled to a pre-track demodulation unit for retrieving thephysical addresses and other control information from the pre-trackpattern constituted by wobble modulation or pre-pits. The main scanningsignal 33 is processed by read processing unit 30 of a usual typeincluding a demodulator, deformatter and output unit to retrieve theinformation. The focus extension unit 32 is described below withreference to FIG. 4.

In an embodiment the device is provided with recording means forrecording information on a record carrier of a re-writable type, forexample DVD+RW. The recording means cooperate with the head 22 andfront-end unit 31 for generating a write beam of radiation, and comprisewrite processing means for processing the input information to generatea write signal to drive the head 22, which write processing meanscomprise an input unit 27, a formatter 28 and a modulator 29. Forwriting information the beam of radiation is controlled to createoptically detectable marks in the recording layer. The marks may be inany optically readable form, e.g. in the form of areas with a reflectioncoefficient different from their surroundings, obtained when recordingin materials such as dye, alloy or phase change material, or in the formof areas with a direction of polarization different from theirsurroundings, obtained when recording in magneto-optical material.

Writing and reading of information for recording on optical disks andformatting, error correcting and channel coding rules are well-known inthe art, e.g. from the CD or DVD system. In an embodiment the input unit27 comprises compression means for input signals such as analog audioand/or video, or digital uncompressed audio/video. Suitable compressionmeans are described for video in the MPEG standards, MPEG-1 is definedin ISO/IEC 11172 and MPEG-2 is defined in ISO/IEC 13818. The inputsignal may alternatively be already encoded according to such standards.

The control unit 20 controls the recording and retrieving of informationand may be arranged for receiving commands from a user or from a hostcomputer. The control unit 20 is connected via control lines 26, e.g. asystem bus, to the other units in the device. The control unit 20comprises control circuitry, for example a microprocessor, a programmemory and interfaces for performing the procedures and functions asdescribed below. The control unit 20 may also be implemented as a statemachine in logic circuits.

In an embodiment the device has a calibration unit 36 coupled to thecontrol unit for calibrating the focus servo system under the conditionsas described below with reference to FIG. 4. In an embodiment thecalibrating function is implemented in a computer program for a PChaving a recording unit, e.g. a DVD+RW drive.

FIG. 3 shows a sensor signal of a focus sensor. The sensor signal valuein Volt is given in vertical direction, and a displacement of thefocusing element 34 in μm in horizontal direction. In an optical storagedrive sensors that measure the focus offset usually have a nonlinearcharacteristic as shown in FIG. 3, which is referred to as “s-curve” 38.A linear region 39 has a substantial proportional relation between thesensor signal and the displacement, and control systems of objectivelens actuator in the optical drive usually operate in this linear regionof the sensor. In the case of an external shock applied to the opticaldrive, the objective lens 34 will start moving with respect to the disc.As long as this displacement stays in the linear region, the controlsystem will force the objective lens back to its correct position. Ifthe displacement of the objective lens exceeds the boundaries of thelinear region, the displacement can no longer be reconstructed from thesensor output so the control system has to be disabled and deflection ofthe objective lens gets disproportionately large. The linear region ofthe s-curve depends on the optical layout of the optical pick up unit ofa drive. In a small form factor drive the linear region 39 happens to bevery small order of ±2 μm), hence this drive is very sensitive toshocks.

For extending the operational range of the focus servo system the tailparts of the s-curve 38 need to be included. In the tail parts therelation between displacement and sensor signal is inverted. However ingeneral the s-curve cannot be inverted, because a sensor output signalvalue may correspond to different displacements. Thereto the devicecomprises a multitude of piecewise converters. Each converter is forconverting a range of the sensor signal values into a position signal,assuming the displacement is within a range corresponding to theconversion range. Further the device has a selection unit for selectingone of the position signals as the displacement signal based on anestimated displacement. The estimated displacement is determined byusing a model of the displacements of the focusing element, for examplean extrapolator that determines a velocity of the displacement based onprevious values of the displacement. In an embodiment other signals areused for the estimation, e.g. signals from a separate shock sensor orsignals based on the periodic nature of a rotating disc having warp ortilt. In an embodiment the estimated displacement is based on a separatedisplacement signal based on a total amount of reflected radiation. Thetotal amount of reflected radiation may for example be determined bysumming the sensor signals of all available partial detectors in aquadrant detector.

FIG. 4 shows a focus extension unit. The optical sensor 41 generates asensor signal that depends nonlinearly on the displacement of thefocusing element, for example the objective lens. The sensor signal isfed into each of three piecewise converters 42, 43, 44. The piecewiseconverters generate three candidate position signals 40 that representthe position of the objective lens. A selector unit 46 compares thethree candidates with an estimated value and selects the signal that isclosest to the estimated value provided by estimator 47. The estimator47 extrapolates some samples from the past. Basically it includes asimple model of the moving focusing element, for example that duringthese few sample times the velocity is constant. The operation of thefocus extension unit is explained with reference to FIGS. 5 to 8.

In an embodiment the estimator calculates a next estimate based on anumber of preceding samples. The output of the estimator at instant k iscalled y[k], and the input u[k]. A basic extrapolator calculates the newvalue:y[k]=2*u[k−1]−u[k−2]In digital signal processing preceding samples are denoted by apolynomial using delay unit z⁻¹, hence u[k−n]=z^(−n)*u[k]. The functionof the estimator is noted as:y=(2z−1)/z ² *u.

In an embodiment the estimator has a calculation based on the first andthird preceding sample. The function for digital processing is noted asfollows:y=(2z ²−1)/z ³ *u.

In an embodiment the focus extension unit comprises a filter unit 45.The three candidate position signals 40 are filtered by the filter 45 tosuppress noise. Three corresponding filtered position signals areprovided to the selection unit 46 for comparing to the estimateddisplacement. A corresponding non-filtered version of the selectedposition signal is then passed as displacement signal 49 to the focuscontroller, while the filtered signal 48 is used to calculate a newestimate. Although the filtered signal might be used for thedisplacement signal, the non-filtered version is usually preferredbecause filtering incurs phase shifting, which may disturb the stabilityof the control loop.

In an embodiment the piecewise converters 42,43,44 comprise a lookuptable. The lookup table provides position values for a large number ofsensor signal values, a different table being provided for eachconverter for the relevant part of the s-curve. For example the firstconverter 42 has a range of position values corresponding to the linearpart 39 of the s-curve. The sensor voltages are the input values for thelookup table, while the position (or a corresponding voltage) is theoutput of the lookup table. The values in the lookup table aredetermined by a calibration measurement of the s-curve. The calibrationmeasurement may be performed during design of the device, or for anindividual device during manufacture.

In an embodiment the piecewise converters 42,43,44 comprise a look-uptable which is filled during a calibration process in the device itself.In the process the focusing actuator is fed by a linearly increasingcontrol voltage, which results in a proportional change in the positionof the focusing element. During the increase the sensor voltage ismeasured. The measured values are subdivided in a number of ranges, onerange for each converter. Within a single range there are no overlappingsensor signal values, but overlap between the ranges occurs due to theshape of the s-curve. The calibration process may be performed for eachrecord carrier after insertion, and/or during use in regular intervals,for example during idle time when no user data access is required.

FIG. 5 shows displacement of an objective lens. Horizontally time isgiven as a number of samples, and vertically the displacement in μm. Thedisplacement 51 exceeds the linear region of the s-curve, so theresulting output voltage of the optical sensor is no longer within thelinear range and therefore not a suitable input signal for a traditionalfocus controller.

FIG. 6 shows a sensor signal. A sensor voltage 61 is given vertically,and time horizontally. The corresponding displacement of the focusingelement is given in FIG. 5. In a central linear part 62, roughly fromtime=45 samples to time=55 samples, the curve has a proportionalrelation between displacement and sensor voltage. In a first part 63 thesensor signal has an inverted relation to the displacement, overlappingwith the linear part. In a last part 64 the sensor signal also has aninverted relation to the displacement, overlapping with the linear part.The sensor signal is fed into the three piecewise converters, whichresults in the three signals shown in FIG. 7.

FIG. 7 a shows position signals for the tail parts of the s-curve. Afist position signal 71 is produced by the third converter 44. Inparticular a first part 72 shows a correct translation of the sensorvoltage of the first part 63 of the s-curve to a displacement. A secondposition signal 74 is produced by the second converter 43. In particulara last part 73 shows a correct translation of the sensor voltage of thelast part 64 of the s-curve to a displacement.

FIG. 7 b shows a position signal for the central part of the s-curve. Aposition signal 75 is produced by the first converter 42. In particulara central part 75 shows a correct translation of the sensor voltage ofthe central part 62 of the s-curve to a displacement.

FIG. 7 c shows a combination of position signals. A first part 72 of theposition signal is connected to a central part 75, and continued withlast part 73 of the position signals provided by the differentconverters. At the crossover points 76,77 the selector decides whichsignal comes closest to the estimated value. Hence the inputdisplacement is reconstructed by the combined signal pieces.

FIG. 8 shows a reconstructed displacement signal. A displacement signal80 is reconstructed by combining the position output signals from theconverters as shown in FIG. 7. The reconstructed displacementcorresponds to the input displacement shown in FIG. 5.

Although the invention has been mainly explained by embodiments usingoptical discs, the invention is also suitable for other record carrierssuch as rectangular optical cards, magneto-optical discs or any othertype of information storage system that requires control of a focuselement. It is noted, that in this document the word ‘comprising’ doesnot exclude the presence of other elements or steps than those listedand the word ‘a’ or ‘an’ preceding an element does not exclude thepresence of a plurality of such elements, that any reference signs donot limit the scope of the claims, that the invention may be implementedby means of both hardware and software, and that several ‘means’ or‘units’ may be represented by the same item of hardware or software.Further, the scope of the invention is not limited to the embodiments,and the invention lies in each and every novel feature or combination offeatures described above.

1. Device for scanning a track on a record carrier via a beam ofradiation, the track comprising marks representing information, thedevice comprising a head (22) for providing the beam and generating atleast one sensor signal, the head comprising a focusing element, afocusing unit (25) for controlling the focusing element (34) to maintainan in-focus position for generating a focused spot on the track independence on a displacement signal based on the sensor signal, thedisplacement signal being indicative of a displacement of the focusingelement with respect to an in-focus position, the sensor signal havingoverlapping ranges of sensor signal values for different ranges of thedisplacement, wherein a multitude of piecewise converters (42,43,44),each converter converting a range of the sensor signal values into afirst, second and third position signal, and selection means (46) forcomparing the first, second and third position signals with an estimateddisplacement signal and for selecting one of the first, second or thirdposition signals as the displacement signal based on an estimateddisplacement.
 2. Device as claimed in claim 1, wherein the selectionmeans (46) comprise an estimator (47) for determining the estimateddisplacement based on previous values of the displacement signal. 3.Device as claimed in claim 1, further comprising a filter unit (45),coupled to an output of said piecewise converters (42, 43, 44) an to aninput of said selection means (46) for providing a multitude of filteredposition signals based on the multitude of position signals, inparticular the filter being a smoothing filter.
 4. Device as claimed inclaim 1, wherein at least one of the piecewise converters (42,43,44) hasa substantially proportional relation between the range of the sensorsignal values and the position signal, and at least one of the piecewiseconverters has a substantially inverse relation between the range of thesensor signal values and the position signal.
 5. Device as claimed inclaim 1, wherein at least one of the piecewise converters (42,43,44)comprises a conversion table for converting the range of the sensorsignal values to the position signal.
 6. Device as claimed in claim 1,wherein at least one of piecewise converters (42,43,44) comprises aconversion table for directly converting a number of sensor signalvalues in the range of the sensor signal values to the position signal,and an interpolator for converting sensor signal values that areintermediate in between said directly converted sensor signal values tothe position signal.
 7. Device as claimed in claim 1, wherein the devicecomprises a control unit (20) including a calibration unit (36) forperforming a focus calibration in which the focusing element iscontrolled to at least one predetermined position and parameters of thepiecewise converters and/or of the selection means are adjusted to thesensor signal corresponding to said predetermined position.
 8. Method ofscanning a track on a record carrier via a beam of radiation, the trackcomprising marks representing information, the method comprisinggenerating at least one sensor signal, controlling a focusing element tomaintain an in-focus position for generating a focused spot on the trackin dependence on a displacement signal based on the sensor signal, thedisplacement signal being indicative of a displacement of the focusingelement with respect to an in-focus position, the sensor signal havingoverlapping ranges of sensor signal values for different ranges of thedisplacement, converting a multitude of ranges of the sensor signalvalues into a multitude of position signals, comparing the multitude ofposition signals with an estimated displacement signal; and selectingone of the position signals as the displacement signal based on saidcomparison.
 9. Device for scanning a track on a record carrier via abeam of radiation, the track comprising marks representing information,the device comprising a head (22) for providing the beam and generatingat least one sensor signal, the head comprising a focusing element, afocusing unit (25) for controlling the focusing element (34) to maintainan in-focus position for generating a focused spot on the track independence on a displacement signal based on the sensor signal, thedisplacement signal being indicative of a displacement of the focusingelement with respect to an in-focus position, the sensor signal givingoverlapping ranges of sensor signal values for different ranges of thedisplacement, wherein a multitude of piecewise converters (42, 43, 44),each converter a range of the sensor signal values into a first, secondand third position signal, and selection means (46) for comparing thefirst, second and third position signals with an estimated displacementsignal and for selecting one of the first, second or third positionsignals as the displacement signal based on an estimated displacement,wherein the selection means (46) comprises an estimator (47) fordetermining the estimated displacement based on previous values of thedisplacement signal, and wherein the estimator (47) calculates as theestimated displacement y=(2z²−1)/z³; z⁻¹ indicating a delay unit indigital signal processing.
 10. Device as claimed in claim 9, furthercomprising a filter unit (45), coupled to an output of said piecewiseconverters (42, 43, 44) an to an input of said selection means (46) forproviding a multitude of filtered position signals based on themultitude of position signals, in particular the filter being asmoothing filter.
 11. Device as claimed in claim 9, wherein at least oneof the piecewise converters (42, 43, 44) has a substantiallyproportional relation between the range of the sensor signal values andthe position signal, and at least one of the piecewise converters has asubstantially inverse relation between the range of the sensor signalvalues and the position signal.
 12. Device as claimed in claim 9,wherein at least one the piecewise converters (42, 43, 44) comprises aconversion table for converting the range of the sensor signal values tothe position signal.
 13. Device as claimed in claim 9, wherein at leastone of piecewise converters (42, 43, 44) comprises a conversion tablefor directly converting a number of sensor signal values in the range ofthe sensor signal values to the position signal, and an interpolator forconverting sensor signal values that are intermediate in between saiddirectly converted sensor signal values to the position signal. 14.Device as claimed in claim 9, wherein the device comprises a controlunit (20) including a calibration unit (36) for performing a focuscalibration in which the focusing element is controlled to at least onepredetermined position and parameters of the piecewise converters and/orof the selection means are adjusted to the sensor signal correspondingto said predetermined position.